High frequency coaxial transmission line for supporting a field emission cathode x-ray tube



y 4. 1967 F. J. GRUNDHAUSER ETAL 3,329,815

HIGH FREQUENCY COAXIAL TRANSMISSION LINE FOR SUPPORTING A FIELD. EMISSION CATHODE X-RAY TUBE 2 Sheets-Sheet l Fil Aug l""24, 1964 FHA/VKJ GRUNDHAUSER M/CHAEL P. HAFNER COL/NGPOWERS //VVENTOR5.

BUCK/ OHM BLORE, KLAROU/ST 8 SPAR/(MAN July 4. 1967 F. J. GRUNDHAU SER ETAL 3,329,816 HIGH FREQUENCY COAXIAL TRANSMISSION LINE FOR SUPPORTING A FIELD EMISSIUN CATHODE X-RAY TUBE Filed Aug. 24, 1964 2 Sheets-Sheet 2 FRA/VkJ GRUNDHA USER MICHAEL I? HAF/VER' COL/N6 POWERS BUCKHO/P/V, FLO/7E, KL/JROU/ST a SPAR/(MAN ATTORNEYS United States Patent Ofitice 3,329,816 Patented July 4, 1967 3,329,816 HIGH FREQUENCY COAXIAL TRANSMISSION LINE FOR SUPPORTING A FIELD EMISSION CATHODE X-RAY TUBE Frank J. Grundhauser, Michael P. Hafner, and Colin G.

Powers, McMinnville, Oreg., assignors to Field Emission Corporation, McMinnville, reg., a corporation of Oregon Filed Aug. 24, 1964, Ser. No. 391,661 8 Claims. (Cl. 250-92) The subject matter of the present invention relates to support structures which also form transmission lines for high frequency electrical signals, and in particular to a rotatable support structure for an X-ray tube that enables movement of such tube into any desired position, while also forming a coaxial transmission line of substantially uniform characteristic impedance for transmitting high frequency electrical pulses to energize such X-ray tube through such support structure with substantially no distortion.

Briefly, one embodiment of the support structure of the present invention includes an outer conductor formed by a plurality of rigid tubular sections joined together by rotatable electrical connections, and an inner conductor formed by a plurality of sections also joined together by other rotatable connections. A plurality of insulating spacers are provided for supporting the inner conductor coaxially within the outer conductor to form a coaxial transmission line of substantially the same uniform characteristic impedance in any position of the support structure. The outer conductor may be filled with an insulating gas and'the rotatable connections of the outer conductor sections provided with gas-tight seals, so that the connections maintain the gas within the outer conductor, maintain electrical connection between such sections and su ort the sections in coaxial relationship in their different rotational positions.

The support structure of the present invention is especially useful when employed to support a high frequency pulsed X-ray tube having a field emission type cathode, which produces narrow pulses of high intensity X-rays by vacuum arc operation. One such X-ray tube is shown in copending U.S. patent application, Ser. No. 289,999, entitled X-Ray Tube Having Field Emission Cathode and Evaporative Anode and Method of Operation, which was filed by W. P. Dyke et al., on June 24, 1963.-

The rotatable support structure of the present invention has several advantages over previous support structures for X-ray tubes, including a coaxial construction which enables it to also function as a transmission line to supply high frequency electrical pulses to energize the X-ray tube. In addition, the present support structure is strong, compact and lightweight so that it may be mounted on a mobile cart, and is also of relatively inexpensive and simple construction. Furthermore, the present support structure employs a plurality of rotatable connections which enable movement of the X-ray tube to any desired position, and a plurality of friction braking devices which hold it in such position.

It is, therefore, one object of the present invention to provide an improved support structure which also functions as a transmission line for high frequency electrical signals.

Another object of the invention is to provide an improved rotatable support structure for an X-ray tube which is strong, compact and lightweight, but of a simple and inexpensive construction.

A further object of the present invention is to provide an improved rotatable support structure for an X-ray tube which also functions as a coaxial transmission line of substantially uniform characteristic impedance for transmitting high frequency electrical pulses to such X-ray tube with substantially no distortion.

An additional object of the present invention is to prO- vide an improved rotatable support structure in which a friction braking device is employed to hold the sections of the support structure in the position to which they are rotated.

Still another object of the invention is to provide a rotatable support structure in which a plurality of rotatable connections are employed that maintain the sections of inner and outer conductors in electrical contact and enable the sections of an inner conductor to be supported coaxially within the outer conductor sections in all rotational positions to provide a coaxial transmission line.

Other objects and advantages of the present invention will be apparent from the following detailed description of a preferred embodiment thereof and from the attached drawings, of which:

FIG. 1 is a plan View of a mobile X-ray apparatus employing a support structure made in accordance with the present invention;

FIG. 2 is a partial sectional view of the support structure of FIG. 1 on an enlarged scale, with the sections of such structure moved into coplanar position; and

FIG. 3 is an enlarged sectional view of a portion of the support structure of FIG. 2 showing the details of the X-ray tube mount and one of the rotatable connections of such structure.

One embodiment of the rotatable support structure of the present invention is shown in FIG. 1 as a mobile support structure 10 mounted on a four-wheeled cart 12, which also supports an electrical pulse generator 14 whose output is connected through the. coaxial cable 16, shown in FIG. 2, to the input of the transmission line formed by such support structure. The input end of the tubular outer conductor of the support structure 10 is connected by means of a screw cap 18 to the top of a housing 19 mounted on the top of the cart to support such support structure on such cart for movement therewith. The tubular support structure 10 may be formed of five sections 20, 22, 24, 26 and 28, which are connected together by four rotatable connections 30, 32, 34 and 36, which enablethe rotation of joining sections through 360 with respect to one another, as indicatedby the arrows adjacent such connections. The plane of rotation provided by connection 30 is always perpendicular to the plane of rotation provided by connection 32, while the plane of rotation of connection 32 is always perpendicular to the plane of rotation of connection 34. Similarly, the plane of rotation of connection 34 is also always perpendicular to the plane of rotation of connection 36. This enables the X-ray tube 38 contained within the output support sec-tion 28, as shown in FIG. 2, to be moved universally into any desired position adjacent the cart 12.

A counterbalance weight 40 in, the form of a solid cylinder of iron or other heavy material may be mounted for sliding movement within one end of the support section 24 in order to counterbalance the X-ray tube and support sections 28 and 26. An adjustable X-ray field limiting collimator (not shown) may be provided within a head 42 attached to the end of the support section 28 adjacent the X-ray tube, as well as a source of visible light and an X-ray transparent mirror for transmitting such light through the adjustable collimator along the same path as the X-ray in order to illuminate the field of the X-ray. The output section 28 of the support structure may be supported by a holder 43 mounted on the top of housing 19 and having a semicircular notch slightly larger than the diameter of such section.

As shown in FIG. 2, each of the support sections 29, 22, 24, 26 and 28 includes a tubular outer conductor 44 having a hollow circular cross-section and which is spaced from an inner conductor 46 also of circular cross-section mounted coaxially within such outer conductor by means of a plurality of insulating spacers 48 of epoxy resin or other suitable insulating material. The tubular support structure of FIG. 2 is provided with a filling of electrically insulating fluid which may be a liquid, such as oil, or a gas, such as sulfur hexafluoride (SP or dichloridifluoromethane, sold by E. I. du Pont de Nemours and Company under the trademark Freon 12, which may be pressurized to one atmosphere. The dielectric constant of either of these gases is about 1, while the dielectric constant of the epoxy resin employed for the insulating spaces 48 is about 4, so that there is a slight non-uniformity in the 90 ohms characteristic impedance of the transmission line formed by the inner conductor 46 and the outer conductor 44. However, the characteristic impedance of the transmission line is still substantially uniform so that there is very little signal reflection within such line. Each of the insulating spacers 48 is provided with a plurality of apertures 50 therethrough radially spaced about the central portion of the spacer which surrounds the inner con-ductor sections, to enable the transmission of the insulating fluid through such spacers. Also each of the rotatable connections 30, 32, 34 and 36 between the adjacent sections of the outer conductor are provided with a gas-tight seal in the manner hereafter described with reference to FIG. 3 in order to prevent the .gas or other dielectric fluid from leaking out of the support structure. It should be noted that the annular double reentrant shape of the insulating spacer 48 enables the electric field to be substantially uniform in the region of such spacer and increases the length of the leakage current path to prevent voltage breakdown along the surface of the spacer.

As shown in FIG. 2, the counterweight 40 slides in and out of a tubular extension 52 which is attached to the outer conductor 44 of section 24 of the support structure but does not communicate with the interior of such outer conductor. A rubber O-ring 54 is provided within an annular notch in and end member 56 attached to one end of the counterweight so that such O-ring engages the inner surface of the tubular projection 52 and holds the counterweight by air entrapment which is controlled by the valve 58 attached to the other end of such counterweight.

The central conductor of coaxial cable 16 is connected to the inner conductor 46 of the input section 20 of the support structure by inserting the end portion 60 of such central conductor into such input section so that a banana plug (not shown) at the tip of such end portion engages a cavity (not shown) in the end of the inner conductor. The shield conductor of such cable is attached to a terminal member 62 which is threaded into an annular, flanged, cup-shaped connector 64. The connector 64 extends through an opening in the top of the housing 19 so that the externally threaded end of such connector is engaged by the internally threaded end of screw cap 18 to attach the support structure to such housing by clamping an annular flange 66 welded to the outer conductor of support section between such screw cap and such connector. Of course, a pair of fluid-tight seals (not shown) must be provided between the flange 66 and the connector 64 as well as between such connector and the terminal member 62, in order to prevent leakage of dielectric gas out of the support structure.

As shown in FIG. 3, the rotatable connection 36 includes :an inner connector member 68 rigidly attached by welding, or the like, to the outer conductor 44 of support section 28 and an outer connector member 70 rigidly attached to the outer conductor 44 of support section 26. Each of these connector members is provided with an annular groove of semicircular cross-section at the mating surfaces of the connector members in order to form a track for ball bearings 72 which enable relative rotation of such connector members. The ball bearings also prevent the connector members 6-8 and from moving axially with respect to one another. It should be noted that the support structure is assembled by first pushing the inner connector member '68 within the outer conductor connector member 70 and then inserting the ball bearings 72 into the track formed between such connector members, by providing a small circular hole (not shown) through the outer connector member in communication with the groove forming the track.

The insulating spacer 48 is attached at its outer surface to the outer conductor member 68 by an epoxy resin adhesive. A rubber O-ring 74 is provided within an annular notch in the outer surface of the inner connector member 68 in order to form a gas-tight seal between the inner and outer connector members while enabling relative rotation between such members. A pair of annular brake rings 76 and 78 are attached by set screws 80 and 82, respectively, to the inner connector member 68 and the outer connector member 70, respectively, so that such brake rings extend outwardly from the outer surface of such connector members. Brake lining members (not shown) are fastened to the brake rings 76 and 78 in a space 83 between such rings so that such brake lining members engage one another. Brake ring 78 is provided with adjustment screws (not shown) which urge its brake lining member axially against the brake lining member on brake ring 76 to increase the braking friction and hold the inner and outer conductor members in the rotational position to which they are set.

The inner conductor 46 of support section 26 is rotatably connected to the inner conductor of the support section 28 by inserting the tubular end of the former over the end of a solid connector terminal 86 attached to the latter inner conductor. A coil spring 88 held within an annular notch on the end of connector terminal 86 makes electrical contact between such inner conductors. Thus, the end portion of reduced diameter of connector terminal 86 holds the tubular end of the other inner conductor in the space between such end portion and the insulating spacer 48 by a slip-fit which enables relative rotation of the inner conductor sections.

The X-ray tube 38 is shown in greater detail in the above referred to copending patent application Ser. No. 289,999, but briefly includes an anode 90 having a conical target portion 92 and a plurality of field emission cathodes 94 each in the form of a large number of sharp pointed needles extending radially inward toward such target. The cathodes and anode are mounted within an evacuated envelope including glass envelope portion 96 through which the anode lead extends into a connection socket at the output end of the inner conductor 46 of support section 28. The field emission cathodes are mounted on a circular metal cup envelope portion 98 having an annular outwardly extending flange 100 welded or otherwise secured thereto. The tube flange 100 is clamped between a tubular metal collimating member 102 and an annular metal connector member 104 having leaf spring fingers in engagement with such flange to ground the cathode 94 of the tube. The connector member 104 is threaded within an annular support ring 106 of metal which is attached to the outer conductor 44 of the support section 28 by screws 108 and is secured to a tubular support member 110 of plastic by screws 112 which extend to a flange portion of such tubular member on which the seal member 104 is supported. The support ring 106 has an annular channel in the outer end thereof to provide spaced concentric inner and outer wall portions. The inner wall portion of the support ring is internally threaded for engagement with external threads on collimator member 102 so that one end of such collimator member engages the flange 100 on the X-ray tube and pushes it against the connector member 104 to hold such X-ray tube in position and a rubber O-ring (not shown) may be employed to provide a gas-tight seal between the collimator flange and the inner wall portion of the support ring. A pair of annular lead shields 114 and 116 may be provided around support member 110 and about the inner wall portion of support ring 106 to limit the transmission of X-rays out of the tube support to the aperture through the collimator member 102. In addition, a rubber O-ring 118 is provided in an annular notch in the outer surface of shield member 106 for engagement with the inner surface of the outer conductor 44 to provide a gas-tight seal therebetween.

When narrow, fast rising electrical pulses of high voltage and high current are supplied from pulse generator 14 through the coaxial transmission line of the support structure to the X-ray tube 38, electrons are emitted from some of the needles of the field emission cathodes 94 and bombard the X-ray target 92 to emit a pulse of high intensity X-rays therefrom which are transmitted out of the X-ray tube and through the collimator 102. The metal envelope portion 98 of the X-ray tube may be provided with a thin X-ray transparent window 120 of beryllium, aluminum, or the like, while the collimator member 102 is provided with an Xray transparent window 122 of plastic which forms a gas-tight seal to retain the dielectric gas within the support structure 10 and prevents dust and other foreign matter frombeing deposited upon the tube window. In addition, a ring 124 of lead may be securcd'to an internal flange within the aperture through the collimator member 102, in order to restrict the field of the X-rays emitted from such collimator.

From' the above it can be seen that the tubular rotatable support structure 10' of the present invention also functions as a transmission line having a substantially uniform characteristic impedance which matches that of the pulse generator 14 in order to transmit elecextremely high intensity due to the vacuum arc operation which enables extremely large amounts of current to flow between the anode and cathode of such tube by producing positive ions of vaporized cathode metal to neutralize the space charge ordinarily surrounding such cathode. Such rotatable support enables the positioning of the X-ray tube in any desired location adjacent the cart on which the tube is mounted, by means of the rotatable connections which maintain good electrical contact between the conductor sections, hold the inner and outer conductors of the transmission line in a uniformed spaced relationship, and maintain a gas-tight seal between the sections of outer conductor.

It will be obvious to those having ordinary skill in the art that various changes may be made in the details of the above described preferred embodiment of the present invention without departing from the spirit of such invention. Therefore, the scope of the present invention should only be determined by the following claims.

We claim: g 1. X-ray apparatus having a coaxial high frequency signal transmission line and rotatable support structure, comprising:

an X-ray tube having a field emission cathode; pulse generator means for producing narrow pulses of high voltage and high current; an outer conductor formed by a plurality of tubular sections; an inner conductor formed by a plurality of sections equal in number to the sections of said outer conductor; means for supporting said inner conductor coaxially within said outer conductor and for insulating the outer conductor sections from the inner conductor sections to form a coaxial high frequency signal transmission line of substantially uniform character- 6 istic impedance which is connected between said pulse generator and said X-ray tube; and

a plurality of connection means for securing said outer conductor sections together and said inner conductor sections together by a plurality of rotatable connections which enable the rotation of said sections in a plurality of planes while maintaining the inner conductor sections in axial alignment with the outer conductor sections and providing said connections with an impedance substantially equal to said characteristic impedance.

2. X-ray apparatus having a coaxial transmission line and rotatable support structure, comprising:

an X-ray tube having a field emission cathode;

pulse generator means for producing narrow pulses of high voltage and high current;

an outer conductor formed by a plurality of tubular sections;

an inner conductor formed by a plurality of sections equal in number to the sections of said outer conductor;

means for supporting said inner conductor coaxially within said outer conductor and for insulating the outer conductor sections from the inner conductor sections to form a transmission line having a substantially uniform characteristic impedance which is connected between said pulse generator and said X-ray tube;

a plurality of connection means for securing said outer conductor sections together and said inner conductor sections together by a plurality of rotatable connections which enable the rotation of said sections in a plurality of mutually perpendicular planes While maintaining the inner conductor sections in alignment with the outer conductor sections and providing said connections with an impedance substantially equal to said characteristic impedance; and

a plurality of seal means for providing a fluid-tight seal at each of the rotatable connections between the outer conductor sections to enable an insulating fluid to be contained within said outer conductor.

3. X-ray apparatus having a rotatable support structure for an X-ray tube, comprising:

pulse generator means for producing narrow electrical pulses of short rise time;

an outer conductor formed by a plurality of tubular sections;

an inner conductor formed by a plurality of sections equal in number to the sections of said outer conductor;

means for supporting said inner conductor coaxially within said outer conductor and for insulating the outer conductor sections from the inner conductor sections to form a transmission line having a substantially uniform characteristic impedance whose input end is connected to the output of said pulse generator means;

an X-ray tube having a field emission cathode and an anode mounted within one of the sections of said outer conductor and having its anode and cathode electrodes connected between the outputs of said inner conductor and said outer conductor; and

a plurality of connection means for securing said outer conductor sections together and said inner conductor sections together by a plurality of rotatable connections which enable the rotation of said sections in a plurality of mutually perpendicular planes while maintaining the inner conductor sections in alignment with the outer conductor sections and providing said connections with an impedance substantially equal to said characteristic impedance.

4. X-ray apparatus having a coaxial transmission line and rotatable support structure for an X-ray tube, comprising:

pulser means for producing high frequency electrical pulses;

an outer conductor formed by a plurality of tubular sections;

an inner conductor formed by a plurality of sections equal in number to the sections of said outer conductor;

means for supporting said inner conductor coaxially within said outer conductor and for insulating the outer conductor sections from the inner conductor sections to form a coaxial transmission line having a substantially uniform characteristic impedance whose input end is connected to the output of said pulser means;

an X-ray tube having a plurality of field emission cathodes and a common anode mounted within one of the sections of said outer conductor so that said anode and said cathodes are connected between the outputs of said inner conductor and said outer conductor;

a plurality of connection means for securing said outer conductor sections together and said inner conductor sections together by a plurality of rotatable connections which enable the rotation of said sections in at least three mutually perpendicular planes while maintaining the inner conductor sections in axial alignment with the outer conductor sections and providing said connections with an impedance substantially equal to said characteristic impedance; and

a plurality of seal means for providing a fluid-tight seal at each of the rotatable connections between the outer conductor sections to enable an insulating fluid to be contained within said outer conductor.

5. X-ray apparatus, comprising:

pulse generator means for producing narrow, fast rising high frequency electrical pulses of high voltage and high current;

an X-ray tube having a field emission cathode structure and an anode;

support means for connecting said pulse generator means to said X-ray tube to transmit said electrical pulses to said tube with substantially no distortion to cause X-ray pulses to be emitted therefrom, and for adjustably supporting said tube in different positions to enable said X-ray pulses to be directed along different paths;

said support means including a plurality of outer conductor sections and a plurality of inner conductor sections equal in number to said outer conductor sections and supported within said outer conductor sections to form a coaxial transmission line having a substantially uniform characteristic impedance; and

a plurality of connection means for securing said outer conductor sections together and said inner conductor sections together by at least one rotatable connection which enables the rotation of said sections while maintaining the sections electrically connected together and providing said connection with an impedance substantially equal to said characteristic impedance.

6. X-ray apparatus, comprising:

pulse generator means for producing narrow electrical pulses of high voltage and high current having short rise times;

an X-ray tube having at least one field emission cathode and an anode;

support means for connecting said pulse generator means to said X-ray tube to transmit said electrical pulses to said tube with substantially no distortion to cause X-ray pulses to be emitted therefrom and for adjustably supporting said tube in different positions to enable said X-ray pulses to be directed along different paths;

said support means including a plurality of tubular outer conductor sections and a plurality of inner conductor sections equal in number to said outer conductor sections and supported within said outer conductor sections to form a transmission line having a substantially uniform characteristic impedance;

plurality of annular insulating spacers surrounding the inner conductor sections and positioned within the outer conductor sections; and

a plurality of connection means for securing said outer conductor sections together and said inner conductor sections together by a plurality of rotatable connections which enables the rotation of said sections while maintaining the sections electrically connected together and providing said connections with an impedance substantially equal to said characteristic impedance.

7. X-ray apparatus, comprising: pulse generator means for producing narrow electrical pulses of high voltage and high current having short rise times;

an X-ray tube having an anode and a plurality of field emission cathodes in the form of sharp pointed needles;

support means for connecting said pulse generator said support means including a plurality of tubular outer conductor sections and a plurality of inner conductor sections equal in number to said outer conductor sections and supported within said outer conductor sections to form a coaxial transmission line having a substantially uniform characteristic impedance, said X-ray tube being contained within one of said outer conductor sections;

a plurality of connection means for securing the sections of said outer conductor together and the sections of said inner conductor together by a plurality of rotatable connections which enable the rotation of said sections while maintaining the sections electrically connected together and providing said connections with an impedance substantially equal to said characteristic impedance;

a plurality of seal means for forming a gas-tight seal between the outer conductor sections at each rotatable connection to enable a dielectric gas to be contained within said outer conductor; and

counterbalancing means attached to said outer conductor including an adjustable weight for counterbalancing said X-ray tube and some of said inner and outer conductor sections.

8. Mobile X-ray apparatus, comprising: pulse generator means for producing narrow pulses of high voltage and high current having short rise times;

an X-ray tube having a field emission cathode and an anode;

support means for connecting said pulse generator means to said X-ray tube to transmit said electrical pulses to said tube with substantially no distortion to cause X-ray pulses to be emitted therefrom and for adjustably supporting said tube in different positions to enable said X-ray pulses to be directed along different paths,

said support means including a plurality of tubular outer conductor sections and a plurality of inner conductor sections equal in number to said outer conductor sections and supported within said outer conductor sections to form a coaxial transmission line having a substantially uniform characteristic impedance, said X-ray tube being contained within one of said outer conductor sections;

a plurality of connection means for securing the sections of said outer conductor together and the sec- 9 10 tions of said inner conductor together by a plurality References Cited of rotatable connections which enable the rotation UNITED STATES PATENTS of said sections while maintaining the sections electrically connected together and in axial alignment 1687509 10/1928 Plepe? et a] 250 91 1,870,959 8/1932 Morrison 25086 and providing said connections with an lmpedance 5 substantiall e ual to said characteristic im edance' 2588436 3/1952 Vlolette 174-21 Y a P 3,082,322 3/1963 Koerner et a1 250-91 and a wheeled cart for carrying said pulse generator means, RALPH NILSON, primary Examiner.

said X-ray tube and said support means with the end of one of the outer conductor sections of said support 10 ARCHIE BORCHELT Examlwr' means being secured to said cart. A. L. BIRCH, Assistant Examiner. 

1. X-RAY APPARATUS HAVING A COAXIAL HIGH FREQUENCY SIGNAL TRANSMISSION LINE AND ROTATABLE SUPPORT STRUCTURE, COMPRISING: AN X-RAY TUBE HAVING A FIELD EMISSION CATHODE; PULSE GENERATOR MEANS FOR PRODUCING NARROW PULSES OF HIGH VOLTAGE AND HIGH CURRENT; AN OUTER CONDUCTOR FORMED BY A PLURALITY OF TUBULAR SECTIONS; AN INNER CONDUCTOR FORMED BY A PLURALITY OF SECTIONS EQUAL IN NUMBER TO THE SECTIONS OF SAID OUTER CONDUCTOR; MEANS FOR SUPPORTING SAID INNER CONDUCTOR COAXIALLY WITHIN SAID OUTER CONDUCTOR AND FOR INSULTING THE OUTER CONDUCTOR SECTIONS FROM THE INNER CONDUCTOR SECTIONS TO FORM A COAXIAL HIGH FREQUENCY SIGNAL TRANSMISSION LINE OF SUBSTANTIALLY UNIFORM CHARACTERISTIC IMPEDANCE WHICH IS CONNECTED BETWEEN SAID PULSE GENERATOR AND SAID X-RAY TUBE; AND A PLURALITY OF CONNECTION MEANS FOR SECURING SAID OUTER CONDUCTOR SECTIONS TOGETHER AND SAID INNER CONDUCTOR SECTIONS TOGETHER BY A PLURALITY OF ROTATABLE CONNECTIONS WHICH ENABLE THE ROTATION OF SAID SECTIONS IN A PLURALITY OF PLANES WHILE MAINTAINING THE INNER CONDUCTOR SECTIONS IN AXIAL ALIGNMENT WITH THE OUTER CONDUCTOR SECTIONS AND PROVIDING SAID CONNECTIONS WITH AN IMPEDANCE SUBSTANTIALLY EQUAL TO SAID CHARACTERISTIC IMPEDANCE. 